The present invention relates to a process for the separation of polyolefins, manufactured by low-pressure polymerization with the aid of catalysts such as those comprising a transition metal compound and an organometallic activator, from the reaction mixture resulting from the polymerization.
It is known that alpha-olefins can be polymerized, for example, in the presence of catalysts based on transition metal compounds and organometallic compounds, which are known to those skilled in the art by the name of Ziegler catalysts.
Similar so-called superactive catalysts are now available, the catalytic activity of which is sufficiently high to enable the elimination of the formerly indispensable operation which consists in purifying the polymers obtained from the catalyst residues which are present therein, in the low-pressure manufacture of polyolefins such as high-density polyethylene and isotactic polypropylene.
Nevertheless, when employing these superactive catalysts, it remains desirable to deactivate the catalysts which remain in the presence of the unreacted monomer after the actual polymerization state. In fact, if this deactivation is not carried out, a post-polymerization takes place in the installations located downstream from the polymerization chamber.
Since this type of polymerization is generally carried out at a pressure greater than atmospheric pressure, and the reaction mixture resulting from the polymerization is then most frequently subjected to a pressure release in a pressure release zone separate from the polymerization chamber in order to facilitate the recovery of the unreacted monomer, the postpolymerization during this pressure release is a source of serious disadvantages.
In fact, the physico-chemical conditions which prevail during this pressure release are different from those prevailing in the polymerization chamber. In particular, the temperature, the pressure and the concentration of the constituents of the medium are different. This leads to the formation of polymers which by no means have the same characteristics as the manufactured polymer, and heterogeneities of composition, of properties and of surface appearance of the latter are consequently found.
The problem is particularly critical in the very frequent case of polymerization processes which incorporate an agent for regulating the average molecular weight of the polymer (hydrogen being most frequently used for this purpose) and a catalyst which is sensitive to the action of this regulator.
In fact, under these conditions, since the pressure in the pressure release zone is lower than that in the polymerization chamber, the ratio of the concentration of hydrogen to the concentration of the residual monomer is lower in this zone than in the polymerization chamber. If the post-polymerization is not prevented from occurring in the pressure release zone, polymers having a much higher molecular weight than the average molecular weight of the manufactured polymer are formed therein.
The presence of these very high molecular weight polymers renders the polymer unsuitable for the manufacture of products such as films or fibers.
In order to avoid the above-mentioned disadvantages, it has already been proposed to prevent the post-polymerization from occurring in the installations located downstream from the polymerization chamber by adding polymerization inhibitors to the reaction mixture which results from the polymerization and contains the catalyst and the unreacted monomer.
Thus, it has already been proposed to add water to this mixture, as a liquid deactivator, and in particular to add it to the above-mentioned pressure release zone. However, this technique is not satisfactory if the organic medium contains a liquid hydrocarbon diluent. In fact, under these conditions, an organic phase and a separate aqueous phase form, which makes it difficult to recover the polymer correctly by the conventional physical means of separation, such as centrifugation. Moreover, if the catalysts used comprise transition metal halides, this addition of water causes their hydrolysis with the liberation of corrosive acid halides. Attempts have been made to neutralize these acid halides, not by adding water as the deactivator, but by adding an aqueous solution of an alkali metal compound, most frequently sodium hydroxide. However, if the catalyst used comprises an organometallic compound of an amphoteric metal (most frequently an organoaluminum compound), the presence of the alkali metal compound causes the pH of the medium to rise and leads to the precipitation of the amphoteric metal (aluminum) hydroxide, with the consequent formation of crusts in the installations and the presence of a solid phase, together with the above-mentioned organic and aqueous phases, which complicates the recovery of the polymer even further.
An attempt could be made to avoid the presence of two separate liquid phases by not exceeding the solubility of water in the hydrocarbon diluent or by using steam. However, under these conditions, it is not possible to avoid substantial corrosion which appears especially in the installations for recovering the hydrocarbon diluent and the residual monomer.
Other liquid deactivators which are conventionally used for purifying polyolefins from catalyst residues, such as alcohols, diketones, alkylene oxides and the like, cannot be suitable either, in the very case where the activity of the catalyst used is such that it enables this purification to be avoided. In fact, the need to recover these deactivators nullifies the economy achieved by employing this type of catalyst.
Finally, attempts have been made to overcome the disadvantages relating to the use of liquid deactivators by substituting them with gaseous deactivators. However, no really satisfactory results have been recorded with the known gaseous deactivators.
Thus, the introduction of oxygen into the reaction mixture resulting from the polymerization presents dangers, in view of the explosive or flammable mixtures which the latter can create with the residual monomer or with the liquid hydrocarbon diluent. Ammonia imparts an undesirable coloration to the final polymer. Nitrous oxide must be employed in very large amounts in order to effectively prevent the post-polymerization from occurring. Finally, carbon dioxide must also be employed in very large amounts in order to stop the polymerization effectively. Furthermore, its solidification temperature, which is relatively high under pressure, leads to technological difficulties such as blockages during the recovery of the unreacted monomer, in particular when the latter is recovered by distillation under pressure.